Adenosylhomocysteine hydrolase plays a critical role in regulating AdoMetdependent methylations in eukaryotic cells by regulating the ratio of AdoMet/AdoHcy. Several approaches are being used to determine the structure and function of this enzyme. 1) Structure Determination: The enzyme has been cloned from a rat liver and a D. discoideum cDNA library. The amino acid sequence was determined and a putative NAD binding site identified. Comparison of the amino acid sequence of the rat liver with the Dictyostelium enzyme indicates that 74% of the amino acids are identical, demonstrating that the enzyme is highly conserved. The cloned cDNA's have been expressed in E. coli and site-directed mutagenesis of the rat liver enzyme is in progress to determine the function of specific amino acid residues in NAD binding and catalytic activity of the enzyme. The expression of the AdoHcy hydrolase mRNA in different tissues and the genomic organization of the AdoHcy hydrolase gene are under investigation. 2) Biological Effects: A large number of adenosine and adenosylhomocysteine analogs have been examined for their ability to function as inhibitors and/or substrates of S- adenosylhomocysteine hydrolase. In vivo these adenosine analogs can form very potent and specific inhibitors of transmethylation reactions, and these inhibitors have a wide range of biological activities, including antiviral activity against several RNA and DNA viruses, inhibition of leukocyte chemotaxis, and stimulation of cell differentiation.